Multi-phase or multi-channel power conversion and current-mode control are commonly employed techniques for DC-DC power conversion in today's electronics (e.g. personal computer) market. Multi-phase power conversion provides a cost-effective power solution when load currents cannot be readily supported by single-phase converters. In a multi-phase system, the switching of each channel is timed to be symmetrically out of phase with each of the other channels.
However, the electronics market has now evolved to the point that the number of phases or channels required in a multi-phase power regulator exceeds the number that a single integrated circuit (IC) can practically support. As the phase count grows above four, the IC package becomes large, and the spacing between the power-delivery points and the controller IC exceeds a distance that can support low-level signal integrity and noise rejection. Signal problems result in inaccuracy or necessitate added expense in terms of extra components to suppress noise, layout constraints, and reduced phase count.
One commercially employed method attempts to solve part of the problem—excessive package size—by cascading multiple current-mode regulators, and using a separate controller IC to generate a triangular-shaped signal that is supplied in common to all of the current-mode regulators. Each current-mode regulator initiates its cycle at a respectively different, programmable point on the triangular-shaped signal, in order to achieve the necessary phase separation between adjacent channels. Correct phase separation between the different channels is an important component to multi-phase power conversion.
Another proposed scheme also cascades separate current-mode regulators, but uses digital signalling to communicate between channels. Digital signals are not prone to the same kind of signal degradation or noise susceptibility as analog signals, so that there is no practical limit to the number of phases, or to the physical separation between them. Within the context of this digital signalling approach, there is one implementation that supplies a common clock pulse signal to each of the cascaded regulator channels, with the channels deciding which one will respond to the next clock pulse. In another digital implementation, there is no clock signal supplied by the separate controller IC, making the controller IC a very simple low-cost device. Since there is no clock, this is a unique, self-oscillating system.
The two digital implementations described above solve the problems of noise immunity, large package pin count, and layout difficulty; however, they are both subject to the problem of inadvertent frequency doubling. This problem arises when a noise signal inadvertently causes one of the multi-phase regulators to trigger or ‘fire’ at the wrong time (i.e., other than when its control signal tells it to fire). If this happens, it creates the sustained situation in which two regulator channels are always firing simultaneously (or nearly so in the clockless case). This effectively doubles the frequency, thereby placing excessive thermal stress in all of the power components.